This invention relates to a rear projection image display device in which an image displayed on a video source is enlarged by a projection lens and projected for imaging on a rear projection screen from the rear side, a rear projection screen for the image display device, and a method of producing the same.
The rear projection type television, in which an image displayed on a small video source such as a projection-type cathode-ray tube (CRT) is enlarged by a projection lens and projected on a rear projection screen, has recently been applied more and more widely to home and business uses, because of the remarkably enhanced image quality and of the pleasure of powerful presence it gives to the viewers through a large screen.
Where a projection-type CRT is used in the rear projection type television, it has been the common practice to use respective sets of a CRT and a projection lens for three primary colors, i.e., red, green and blue, and combine the three primary color images with each other on the screen, for producing sufficiently bright images on the screen. In order to minimize the variations in the color balance of the red, green and blue primary colors with variations in the position of the viewer or the angle at which the screen is viewed, there has been generally used a construction in which, as for instance described in Japanese Patent Application Laid-Open (KOKAI) No. 58-192022 (1983), a rear projection screen is composed by combining a first sheet member comprising a Fresnel lens with a second sheet member which has a plane of incidence and a plane of emergence each comprising a lenticular lens array and which comprises dispersed therein a particulate material for scattering light.
FIG. 1 shows a perspective view of an important portion of a rear projection screen according to the prior art as mentioned above. In the figure, there are shown the rear projection screen 1, a first sheet member 2, a second sheet member 3, and respective bases 2B and 3B of the first and second sheet members 2 and 3. A flat surface 21 constitutes a plane of incidence of the sheet member 2. A plane of emergence 22 of the first sheet member 2 comprises a Fresnel lens. A plane of incidence 31 of the second sheet member 3 has a configuration in which lenticular lenses with the longitudinal direction thereof set in the vertical direction are arrayed in the horizontal direction. On a plane of emergence 32 of the second sheet member 3, lenticular lenses similar to the above are arrayed substantially opposite to the lenticular lenses on the plane of incidence 31. Light non-transmitting areas 3N (namely, the areas in which light is not transmitted) at the boundary portions of the lenticular lenses have flat surfaces for providing thereon a light absorbing layer, as described below. The light absorbing layer 6 is provided on the light non-transmitting areas 3N. The second sheet member 3 has particulates of a light diffuser, for scattering light, dispersed in the base 3B.
In the conventional rear projection screen as above, a luminous flux emerging from the entire part of the image displayed on the projection-type CRT comes to be incident on the entire part of the plane of incidence while diverging through a projection lens. In this case, the Fresnel lens at the plane of emergence 22 of the first . sheet member 2 of the screen converts the incident luminous flux into a substantially parallel luminous flux, which is incident on the second sheet member 3. The light incident on the second sheet member 3 forms an image at the focus on the plane of emergence 32 under the function of the lenticular lenses on the plane of incidence 31, and the light emerges to the viewer's side while being diffused horizontally from the focus. The resolution of the rear projection screen 1 is enhanced when the configuration of the lenticular lenses on the plane of incidence 31 of the second sheet member 3 is so designed as to reduce, as much as possible, the focus on the plane of emergence 32 of the sheet member 3.
FIGS. 2a and 2b each shows a sectional view of the second sheet member 3 of the conventional rear projection screen 1 mentioned above, wherein FIG. 2a is a vertical sectional view through the center of one lenticular lens, and FIG. 2b is a horizontal sectional view.
In FIGS. 2a and 2b, particulates of a light diffuser are dispersed in the base 3B of the second sheet member 3, as mentioned above, whereby a beam of light 14 incident on the plane of incidence 31 proceeds while being diffused horizontally and vertically, to emerge through the plane of emergence 32 to the viewer's side. An increase in the amount of the light diffuser causes an increase in the angular range of diffusion of light, namely, the so-called viewing angle. As shown in FIG. 2a, however, the incident light beam 14 is diffused by the light diffuser before reaching the focus on the pane of emergence 32. Therefore, the more the amount of the light diffuser is increased for the purpose of enlarging the viewing angle, the more fuzzier the image formed at the focus on the plane of emergence 32, and the greater the spot diameter d at the plane 32, resulting in lower focusing characteristics and poorer resolution.
In the above-mentioned rear projection screen 1 according to the prior art, the plane of incidence 31 of the second sheet member 3 comprises an array of lenticular lenses of which the longitudinal direction is set in the vertical direction of the screen, as mentioned above. The focal plane of the lenticular lenses constitutes the pane of emergence 32 of the second sheet member 3. On the plane of emergence 32, the foci of the lenticular lenses are arranged at the same pitch as the pitch of the lenticular lenses on the plane of incidence of the second sheet member, and light non-transmitting areas 3N through which light is substantially not transmitted are present between the foci. For the purpose of reducing reflection of ambient light such as illuminating light to thereby enhance the contrast of images, therefore, a light absorbing layer 6 is provided on the plane of emergence 32 of the second sheet member 3 in the light non-transmitting areas 3N. The light absorbing layer 6, appearing as if black straight lines in the vertical direction of the screen were arranged in parallel, is generally called "black stripes".
As has been mentioned above, the light incident on the lenticular lens surface of the second sheet member 3 is diffused by the light diffuser before reaching the focus on the plane of emergence 32. Therefore, as shown in FIG. 2b, part 14-1 of the light is absorbed by the light absorbing layer 6, without reaching the focus of the lenticular lens. There has thus been the problem that the quantity of light at a bright image portion is reduced, resulting in a lower contrast.
Besides, in the conventional rear projection screen 1 mentioned above, both the first and second sheet members 2 and 3 are generally provided by using thermoplastic resins for the bases thereof, from the viewpoint of productivity. The thermoplastic resins, however, are inferior to metals and glasses in hardness. Especially, the second sheet member 3 is exposed to the viewer's side, adjacently to a casing of a rear projection image display device; therefore, the second sheet member 3, if formed by use of a thermoplastic resin for the base thereof, would be easily damaged during handling of the display device.